A subsurface or subterranean well transits one or more formations. The formation is a body of rock or strata that contains one or more compositions. The formation is treated as a continuous body. Within the formation hydrocarbon deposits may exist. Typically a wellbore will be drilled from a surface location, placing a hole into a formation of interest. Completion equipment will be put into place, including casing, tubing, and other downhole equipment as needed. Perforating the casing and the formation with a perforating gun is a weft-known method in the art for accessing hydrocarbon deposits within a formation from a wellbore.
Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, several types of tubulars are placed downhole as part of the drilling, exploration, and completions process. These tubulars can include casing, tubing, pipes, liners, and devices conveyed downhole by tubulars of various types. Each well is unique, so combinations of different tubulars may be lowered into a well for a multitude of purposes.
Explosively perforating the formation using a shaped charge is a widely known method for completing an oil well. A shaped charge is a term of art for a device that when detonated generates a focused explosive output. This is achieved in part by the geometry of the explosive in conjunction with a liner in the explosive material. Generally, a shaped charge includes a metal case that contains an explosive material with a concave shape, which has a thin metal liner on the inner surface. Many materials are used for the liner; some of the more common metals include brass, copper, tungsten, and lead. When the explosive detonates the liner metal is compressed into a super-heated, super pressurized jet that can penetrate metal, concrete, and rock.
A perforating gun has a gun body. The gun body typically is composed of metal and is cylindrical in shape. Within a typical gun tube is a charge holder, which is a tube that is designed to hold the actual shaped charges. The charge holder will contain cutouts called charge holes where the shaped charges will be placed.
A perforating gun may have scallops machined on the exterior surface for each shaped charge. The purpose of the scallop is to provide a flat uniform spot for the explosive jet to easily puncture with minimal effect on the direction or strength of the jet. Furthermore, when a perforating gun is pulled out of a hole after detonation, a hole located in a scallop is an obvious sign that the shaped charge behind it exploded. One of the fears in the field is that unexploded ordinance may be brought to the surface. Further, when sending off a gun to be recycled, the recyclers will demand that the gun tube is free from unexploded ordinance. A blasted hole in every scallop is evidence that there is no unexploded ordinance.
In some instances not all of the shaped charge spots are used. There are applications where a limited number of shaped charges are needed. It may result in one or more of the charge holes in the charge tube going unfilled. There are cases were they may be more scallops in the perforating gun than shaped charges. In these situations there will be no perforated holes in the scallops where there are no shaped charges. This presents a problem to the crew pulling the gun out of the hole because there could be confusion as to whether the missing perforation on a scallop is the result of intentionally loading fewer shaped charges or because there is unexploded ordinance still inside the gun. The problem of confusion continues when the perforating gun tube is sent to a recycler. The recycler cannot tell if the absence of a hole in a scallop is because there was no shaped charge behind the scallop or because the shaped charge failed to explode. This uncertainty may cause a recycler to refuse to accept a gun tube.